The problem for BBC Three before was that it needed to show a very high amount of original programming, including news and documentaries when it launched... obviously this must have decreased over the years with the amount of family guy & American dad that was shown.
The 90% figure was new content made in the UK, and as expected it would have an annual quota of hours. Most of this would have only aired before, say, midnight, as the channel, much like BBC Four now, is full of repeats of content aired earlier in the night.
Anyway Family Guy started on Channel 4, then moved to BBC Two before it settled on BBC Three, I don't know why it moved to BBC Three, perhaps it just seemed a better home for it. Think American Dad was the same. There was a pattern to the Family Guy repeats, as I think they could only air them so many times a week, so most double bills ended up repeated later in the week (2 nights gap IIRC, so Monday's episodes appeared again on Wednesday ) and on the weekend, the so-called stacks which was basically an omnibus in everything but name of that week's episodes.